1 What is the overlay analysis?

Overlay analysis permit to merge different layers in a single one. This technique is suitable to identify the best or most preferred locations for a specific phenomenon. Between problems that could be solved using overlay analysis include:

  • Where to site a new childrendarden.
  • Which sites are better for solar power plants.
  • Estimate the vulnerability to landslides.

2 Area of Interest (AOI) an dataset

The AOI is the city of Salzburg, Austria. The dataset was obtained from ArcGIS REST Services Directory, and download using a Python script. After download the shapefiles, a spatialite dataset was created with the following tables:

  • Salzburg City: Linestring which represent the outline of Salzburg city.
  • Districts Salzburg City: District of Salzburg city.
  • ZGIS_SBG_LULC: Land use of Salzburg city.
  • ZGIS_SBG_Geology: Geology of Salzburg city.

3 Filter the land use table

The first step of the process was to filter the table ZGIS_SBG_LULC. For the assignment, I chose the “Industrial, commercial, public, military, and private units” class. See the map below to visualize these areas in Salzburg city.

4 Intersect the filtered area and the geology dataset

In SpatiaLite the spatial relations between two different tables (geometries) are made by the Dimensionally Extended 9-Intersection (DE-9IM) model. The intersection operation is realized in three-levels: interior, boundary, and exterior, and it is necessary to have at least one point in common. The map below displays our results after the intersection.

5 Aggregation by District

Finally, we will aggregate (grouping) the geology into the district table according to the larger area. Since in SpatialLite there is no a tool called ‘Summarize Data’ like ArcGIS. We use a for (loop), ST_INTERSECT, and ST_SUMMARY to obtain a similar tool. The map below show our results.